Light-harvesting rods that absorb intensely across the visible spectrum and funnel the resulting excited-state energy to one end of the rod may find use in molecular-based solar cells (Loewe, R. S. et al., J. Mater. Chem. 2002, 12, 1530-1552). A number of general challenges must be met to achieve effective light-harvesting rods of this type, including (1) spectral matching of the absorption with the incident light, (2) appropriate electronic coupling of the pigments, and (3) solubility of the rods and precursors to the rods. The approach we have employed has focused on covalently linked arrays of porphyrins wherein the linker joining the porphyrins provides weak electronic coupling between the porphyrins. Weak coupling refers to the close similarity of the respective absorption spectra (associated with the first excited singlet state) and electrochemical potentials of the component parts and those of the components upon incorporation in the dyad or oligomer. Weak coupling enables rational design based on knowledge of the properties of the component building blocks (Holten, D. et al., Acc. Chem. Res. 2002, 35, 57-69). Despite the weak coupling, energy transfer in a porphyrin-based light-harvesting array can be exceptionally fast and efficient (del Rosario Benites, M. et al., J. Mater. Chem. 2002, 12, 65-80).
The synthesis of linear multiporphyrin rods has been pursued by methods employing the stepwise incorporation of porphyrin units, by polymerization methods, and by combinations of the two approaches. The stepwise methods generally have afforded arrays comprised of eight or fewer porphyrins along the axis of the rod (Loewe, R. S. et al., J. Mater. Chem. 2002, 12, 1530-1552; del Rosario Benites, M. et al., J. Mater. Chem. 2002, 12, 65-80; Burrell, A. K. et al., Chem. Rev. 2001, 101, 2751-2796). The polymerization methods have afforded a variety of architectures, including pendant (Aota, H. et al., Chem. Lett. 1994, 2043-2046), cofacial (Shimidzu, T. Synth. Met. 1996, 81, 235-241), or backbone polymers (Anderson, H. L. Chem. Commun. 1999, 2323-2330; Yamamoto, T. et al., Macromolecules 2000, 33, 5988-5994; Scamporrino, E. and Vitalini, D. Macromolecules 1992, 25, 1625-1632; Maruyama, H. et al., Synth. Met. 1998, 96, 141-149; Jiang, B. et al., Chem. Commun. 1998, 213-214; Jiang, B. and Jones, W. J. Jr. Macromolecules 1997, 30, 213-214; Jiang, B. et al., Chem. Mater. 1997, 9, 2031-2034; Ferri, A. et al., J. Chem. Soc., Dalton Trans. 1998, 4063-4069). Osuka has employed the combined synthetic approach to prepare linear meso,meso-linked porphyrins containing up to 128 porphyrins (Aratani, N. et al., Angew. Chem. Int. Ed. Engl. 2000, 39, 1458-1462; Aratani, N. and Osuka, A. Macromol. Rapid Commun. 2001, 22, 725-740). The meso,meso-linked porphyrins are very tightly coupled electronically but also present a rare example of a highly soluble long linear polymer of porphyrins (Nakano, A. et al., Chem. Eur. J. 2000, 6, 3254-3271). Regardless of synthetic method, the use of porphyrins in light-harvesting rods typically presents two problems: poor solubility and limited spectral coverage across the solar spectrum.
Poor solubility limits the ability to handle the porphyrin arrays and also crimps the length of rods that can be created. One approach to ameliorate the poor solubility of multiporphyrin arrays has been to suppress cofacial aggregation of the porphyrins by incorporating bulky groups at the meso-positions of the porphyrins, such as 3,5-di-tert-butyl phenyl groups (Crossley, M. J. and Burn, P. L. J. Chem. Soc., Chem. Commun. 1987, 39-40; Tamiaki, H. et al., Bull. Chem. Soc. Jpn. 1993, 66, 2633-2637) or 2,6-disubstituted aryl groups (Lindsey, J. S. and Wagner, R. W. J. Org. Chem. 1989, 54, 828-836; Wagner, R. W. et al., Tetrahedron 1994, 50, 11097-11112; Prathapan, S. et al., J. Am. Chem. Soc. 1993, 115, 7519-7520). The latter approach has led to the use of mesityl-substituted porphyrins (e.g., meso-tetramesitylporphyrin, TMP), which generally have greater solubility than phenyl-substituted porphyrins (e.g., meso-tetraphenylporphyrin, TPP). However, our attempts to use mesityl-substituted porphyrin building blocks (e.g., 1) in polymerizations resulted in insoluble oligomeric products. The replacement of the methyl groups in TMP with ethyl groups (2) led to only marginal changes in solubility. Use of very bulky pentafluorobenzyloxy groups led to increased solubility (3, 4) but still the resulting oligomers, obtained by polymerization alone or in the presence of the capping agent 5, were quite insoluble (Scheme 1).
The limited spectral coverage provided by porphyrins across the solar spectrum is an intrinsic property of the porphyrin chromophore. Porphyrins absorb intensely in the near-UV region but absorb poorly across the remainder of the visible spectrum. By contrast, chlorins and bacteriochlorins both absorb strongly in the blue and red regions of the spectrum. The chlorin chromophore (a dihydroporphyrin) and bacteriochlorin chromophore (a tetrahydroporphyrin) form the basis for chlorophyll and bacteriochlorophyll in green plant and bacterial photosynthesis, respectively. Porphyrins have been widely employed in light-harvesting studies (Burrell, A. K. et al., Chem. Rev. 2001, 101, 2751-2796) because of the close structural similarity yet greater synthetic tractability of porphyrins versus chlorins or bacteriochlorins. One approach to increase the overall absorption efficiency of light-harvesting systems is to employ accessory pigments. A good accessory pigment for porphyrins would absorb light strongly in the trough (430-540 nm) between the porphyrin Soret (B) and Q-bands, transfer energy efficiently to the porphyrin, not engage in electron-transfer quenching with the photoexcited porphyrin, display good solubility in common organic solvents, and provide compatibility with a modular building block approach (Wagner, R. W. and Lindsey, J. S. Pure Appl. Chem. 1996, 68, 1373-1380; Wagner, R. W. and Lindsey, J. S. Pure Appl. Chem. 1998, 70 (8), p. i).
We have previously employed boron-dipyrrin dyes to serve as accessory pigments in porphyrin-based devices, including a molecular photonic wire (Wagner, R. W. and Lindsey, J. S. J. Am. Chem. Soc. 1994, 116, 9759-9760), optoelectronic gates (Wagner, R. W. et al., J. Am. Chem. Soc. 1996, 118, 3996-3997; Ambroise, A. et al., Chem. Mater. 2001, 13, 1023-1034), and light-harvesting arrays (Li, F. et al., J. Am. Chem. Soc. 1998, 120, 10001-10017). Although the boron-dipyrrin dyes display many of the desirable attributes mentioned above, these dyes were found to exhibit two excited-state conformers with rather short lifetimes (˜15 ps, ˜500 ps) (Li, F. et al., J. Am. Chem. Soc. 1998, 120, 10001-10017). The presence of two conformers complicated analysis of the excited-state dynamics of the arrays. We also have employed rhodamine dyes (Lindsey, J. S. et al., Tetrahedron 1994, 50, 8941-8968) and cyanine dyes (Lindsey, J. S. et al., Tetrahedron 1989, 45, 4845-4866). However, both of the latter types of dyes are intrinsically charged, causing severe difficulties in purification. Others have employed carotenoids as accessory pigments (Gust, D. et al., Acc. Chem. Res. 2001, 34, 40-48). However, the very short excited-state lifetime (˜1 ps) requires the carotenoid to be placed in very close proximity to the porphyrin in order to achieve efficient energy transfer.
We have investigated the use of perylene dyes as accessory pigments for porphyrins in perylene-porphyrin dyads (Miller, M. A. et al., J. Org. Chem. 2000, 65, 6634-6649; Prathapan, S. et al., J. Phys. Chem. B 2001, 105, 8237-8248; Yang, S. I. et al., J. Phys. Chem. B 2001, 105, 8249-8258; Yang, S. I. et al., J. Mater. Chem. 2001, 11, 2420-2430). As a general class, perylenes meet the criteria for light absorption in the trough between the porphyrin Soret (B) and Q-bands (Langhals, H. Chem. Ber. 1985, 118, 4641-4645) and have the requisite spectral properties for efficient Förster energy transfer (long fluorescence lifetime (Ford, W. E. and Kamat, P. V. J. Phys. Chem. 1987, 91, 6373-6380), high fluorescence quantum yield (Langhals, H. Chem. Ber. 1985, 118, 4641-4645; Rademacher, A. et al., Chem. Ber. 1982, 115, 2927-2934; Ebeid, E. M. et al., J. Phys. Chem. 1988, 92, 4565-4568), and respectable overlap of the fluorescence emission bands with the absorption bands of a zinc porphyrin). However, achieving efficient energy transfer without competing or subsequent electron-transfer quenching processes requires selection of the appropriate composition of the perylene, architecture of the linker, and perylene-linker attachment sites. In addition, the types of substituents on the perylene and the overall perylene-porphyrin architecture affect the solubility of the construct.
Accordingly, there remains a need for new compounds having appropriate properties for the synthesis of light harvesting rods and light harvesting arrays, and for the construction of solar cells.